Conventionally, each chain gear forming a multi-stage chain gear assembly comprises merely a single gear body having a number of teeth, and a driving chain is moved forcibly axially of the chain gears by operation of the derailleur to switch the chain from one of the plurality of chain gears to any other chain gear, thereby making a speed-change. In this instance, it is necessary to forcibly disengage the chain from a corresponding chain gear in mesh therewith by operation of the derailleur because each chain gear is formed with merely a single gear body. Hence, such device provides poor speed-change efficiency.
Meanwhile, a shifting device has been proposed as shown in U.S. Pat. No. 4,127,038 in which the gear body is formed of a pair of gear sectors each having a partial gear, and each gear sector is swingable, so that a partial gear of one gear sectors is moved to allow the teeth at its associated partial gear to be positioned within a plane of the teeth at a partial gear of the other gear sector to thereby switch the chain.
But, the shifting device does not employ a derailleur but instead adapts the gear sectors to be moved so that the shifting device requires a special movement mechanism for moving the gear sectors, and it is necessary to set the partial gear of each sectors to come within a plane of the partial gear of the other gear sector. Additionally, it is necessary to correspond the pitch of the teeth at each partial gear with the chain pitch of the driving chain due to the fact that the shifted chain in mesh with the partial gear upon its movement needs to be surely engaged with the other partial gear. As a result, this shifting device as a whole is complex in construction, troublesome in machining, and expensive to produce. Thus, this shifting device cannot basically solve the aforesaid problem of the conventional shifting device employing the derailleur.